The present invention relates to a process for converting coal directly into predominantly liquid products suitable for making hydrocarbon fuel, by slurrying the comminuted coal in a pasting oil and digesting the slurried coal under hydrogenative conditions at a temperature ranging from about 380.degree. to about 500.degree. C. and a pressure in the range of about 8 MPa (80 bar) to about 30 MPa (300 bar), if, and to the extent necessary, removing non-liquefied solids from the digested slurry, fractionating the digested slurry by distillation to produce a light oil fraction, a middle oil fraction and a heavy or residue fraction, the fraction cutting temperatures (reduced to atmospheric pressure) being about 200.+-.50.degree. C. between the light oil and middle oil and about 400.+-.50.degree. C. between the middle oil and the heavy or residue fraction and recycling part of said fractions to substantially or predominantly provide the pasting oil, and wherein one part of the coal is slurried and digested in a first stream (I) wherein the pasting oil comprises heavy or residue fraction, including a heavy or residue fraction derived from coal slurried and digested in a separate and distinct second stream (II).
A process of the aforesaid type, wherein the pasting oil comprises light oil in addition to the recycled heavy or residue fraction is proposed in earlier patent application Ser. No. 969,473 of the same assignee, now U.S. Pat. No. 4,251,346 and corresponding patents and patent applications in other countries, which are not to be considered as prior publications, but which by reference thereto are to be considered as part of the present disclosure. As described in those earlier applications, there exists a long and technologically important history of prior art which accordingly need not be repeated here.
These prior processes serve to produce products ranging from compounds which at room temperature are solid, right down to light liquids, usually with at least a substantial proportion of gaseous (and usually less desirable) products. In order to attain satisfactory degrees of extraction and liquefaction, most prior art processes employ hydrogenative conditions. Such hydrogenative conditions may partly or wholly be created by the use of a solvent having pronounced hydrogen donor properties, but are usually due at least in part to the employment of hydrogen under pressure, with or without an extraneously introduced hydrogenation catalyst.
Difficulties have been experienced in prior art processes to achieve adequate self-sufficiency in respect of solvent requirements and/or adequate solids carrying capacities of the solvent. Solvents which do have a desirable solid carrying capacity tend to result in net products mainly consisting of solid or near solid products, there being little or no net product in the highly desirable liquid hydrocarbon region. Increasing the hydrogenation rate, either in order to improve the depth of extraction or to increase the yield of liquids has sometimes tended to result in excessive gas yields. There exists a need in the art for a convenient process which is capable of producing high extract yields, particularly in the liquid hydrocarbon range, in particular the gasoline and optionally the diesel fuel range with low to modest hydrogen comsumption, very high pressures being avoided. At the same time the main products should be distillates which can be refined to conventional liquid fuels and chemcals without undue difficulty.
Preferred embodiments of the present invention are directed to process configurations capable of converting substantially all the liquefiable coal components to distillate products, whilst being capable optionally to be so modified that part of a distillation residue formed in the process is not recycled but withdrawn as a valuable byproduct having surprisingly superior characteristics as a raw material for making premium electrode coke.
An important advance in the art is proposed in the aforesaid Pat. No. 4,251,346. According to that proposal, the coal is slurried in a solvent system comprising at least 20% by mass of a comparatively low boiling fraction, liquid at room temperature and boiling not higher than 200.degree. C., more than 10% by mass of a heavy or residue fraction, mostly solidifiable at room temperature, but liquid at the digesting temperature and not more than 30% by mass boiling between 200.degree. and 450.degree. C. The temperature for digesting the coal is maintained above the critical temperature of components of the low boiling fraction and the resulting mixture of solvent system and digestion products is fractionated distillatively so as to recover liquid hydrocarbons from such fractionating, those boiling from 200.degree. to 450.degree. C. constituting not less than 50% of the total liquid hydrocarbon net recovery whilst material comprising a fraction boiling below 200.degree. C. and a bottom fraction is recycled to the slurrying stage.
More preferably, in that process the ratio of light oil to heavy residue in the pasting oil is from 3:1 to 1:3, preferably from 2:1 to 1:2. Within the boiling point range of 200.degree. to 450.degree. C. the material distribution pattern of the pasting oil (mass vs temp.) exhibits a gap or at least a pronounced minimum. The preferred digesting temperature is from 400.degree. to 480.degree. C. at a hydrogen partial pressure of from 5 to 25 MPa (50 to 250 bar), preferably in the presence of a hydrogenation catalyst, e.g. molybdenum, tungsten, iron or cobalt sulphides, e.g. introduced by impregnating the coal with amounts equivalent to 0,1 to 10% ammonium molybdate based on the dry mass of the coal.
The aforesaid process as described in the cited earlier patent application is usually conducted as a single stream process. However, the heavy residue fraction may be used wholly or in part as a raw material for making premium electrode coke of exceptionally high quality. In that event there may be a shortfall of heavy fraction for recycling to the pasting oil. Such shortfall is then made up by using heavy residue fraction derived from another, separate and independent coal extraction process stream.
The aforesaid proposal was directed predominantly at the direct production of liquid products boiling in the boiling range of diesel fuels. Although the process has substantial merit, there are disadvantages in that the products in the diesel oil boiling range require further hydrogenation, whereby the final yield of good quality diesel oil is reduced, mostly by the formation of lighter products in the gasoline boiling range. These latter, however, require substantial further treatment such as hydro treatment and platforming in order to meet gasoline specifications.